1. Field of the Invention
The present invention relates to a method of producing a non-oriented electromagnetic steel strip having superior magnetic properties. More particularly, the present invention is concerned with a method of producing non-oriented electromagnetic steel strip which has a high level of magnetic flux density and superior surface appearance.
2. Description of the Related Art
Non-oriented electromagnetic steel sheets are used as materials of cores of rotating machines such as motors, as well as cores of transformers and stabilizers. To improve efficiency of operation of these electrical cores while reducing their sizes it is necessary to raise the level of the magnetic flux density and to reduce the iron loss of the electromagnetic steel sheet used as the core material.
It has been known that one way of improving magnetic properties of non-oriented electromagnetic steel sheets is to coarsen the crystal grains of the steel strip before cold rolling.
The present inventors have proposed, in Japanese Patent Publication (Kokoku) No. 57-35628, a method for coarsening the crystalline structure of an electromagnetic steel strip which is to be cold-rolled, wherein an electromagnetic steel strip, which is to be cold-rolled, is hot-rolled such that the hot-rolling is finished at a temperature not lower than the Ar.sub.3 transformation temperature of the steel which is determined on the basis of the chemical composition of the steel. The hot-rolled steel strip is annealed for at least 30 seconds up to 15 minutes at a temperature not higher than the A.sub.3 transformation temperature.
The inventors also proposed, in Japanese Patent Laid-Open (Kokai) No. 2-182831, a method in which hot-rolling of a steel strip is finished at a temperature not lower than the Ar.sub.3 transformation temperature and the hot-rolled steel strip is held at a temperature not higher than the A3 transformation temperature for 15 to 30 seconds, followed by cooling which is effected at a controlled cooling rate.
In these methods, however, coarsening of the crystal grains cannot be attained satisfactorily particularly when the annealing time is near the shorter end (30 seconds) of the annealing period, resulting in large fluctuation of the magnetic characteristics. Conversely, when the annealing time approaches the longer limit (15 minutes) of the annealing period, the crystalline structure becomes too coarse so that the appearance of the product is impaired due to roughening or wrinkling of its surface.
Japanese Patent Laid-Open (Kokai) No. 58-136718 discloses a method in which a steel strip is hot-rolled down to a final temperature which is within the .gamma.-phase region and not more than 50.degree. C. higher than the Ar.sub.3 transformation temperature, the strip being then taken-up at a temperature which is not higher than the A.sub.3 transformation temperature but not lower than 700.degree. C. so as to coarsen the ferrite crystal grains to a size which is not greater than 100 .mu.m, thereby improving magnetic properties of the steel strip.
Japanese Patent Laid-Open (Kokai) No. 54-76422 discloses a method in which a hot-rolled steel strip is taken up at a temperature ranging between 750.degree. and 1000.degree. C., and is self-annealed by the heat possessed by the steel strip itself, whereby the steel strip is recrystallized to crystal grains sized between 50 and 70.mu.m so as to exhibit improved magnetic characteristics.
These known methods for improving magnetic properties by employing take-up temperatures not lower than 700.degree. C. conveniently eliminate the necessity for annealing but suffer from a disadvantage in that, since the take-up temperature is high, both side edge portions of the coiled steel strip are cooled at a greater rate than the breadthwise central portion of the coil and at a higher speed at the starting and terminating ends of the coil than at the mid portion of the coil, which not only produce nonuniform distribution of magnetic properties over the entire coiled steel strip but also impair the effect of pickling which is conducted for the purpose of descaling.
Japanese Patent Publication (Kokoku) No. 45-22211 discloses a method in which a hot-rolled steel strip is cold-rolled at a rolling reduction of 0.5 to 15% and is then subjected to annealing which is conducted for a comparatively long time at a temperature not higher than the A.sub.3 transformation temperature, so as to coarsen the crystalline structure of the steel strip thereby reducing iron loss. In this method, however, the annealing after cold rolling is conducted in accordance with a so-called box-annealing method at a temperature of 800.degree. to 850.degree. C. for a comparatively long time of 30 minutes to 20 hours (10 hours in all the illustrated examples). Such a long term annealing is undesirable from the viewpoint of cost and tends to cause excessive coarsening to grain sizes of 180 .mu.m or greater, leading to inferior appearance of the product.
Japanese Patent Laid-Open (Kokai) No. 1-306523 discloses a method for producing a non-oriented electromagnetic steel sheet having a high level of magnetic flux density, wherein a hot-rolled steel strip is subjected to cold rolling at a small reduction conducted at a rolling reduction of 5 to 20%, followed by annealing for 0.5 to 10 minutes at a temperature ranging from 850.degree.to 1000.degree. C. Annealing is conducted in a continuous annealing furnace in this case but this method uneconomically requires huge equipment because the annealing has to be completed in a short time, e.g., 2 minutes or so as in the illustrated examples.
All these known methods are intended to improve magnetic properties by coarsening the crystalline structure of the steel strip before the strip is subjected to cold-rolling. Unfortunately, these known methods do not provide sufficient combined magnetic properties, product quality and economy of production.
Japanese Patent Laid-Open Nos. 1-139721 and 1-191741 disclose methods of producing semi-processed electromagnetic steel sheets, wherein skin pass rolling is conducted at a rolling reduction of 3 to 15% as the final step. The skin pass rolling for semi-processed steel strip, however, is intended to control the hardness of the rolled product. In order to assure required magnetic properties the skin pass rolling must be followed by a special annealing which must be conducted for a comparatively long time, e.g., 2 hours, at a temperature of, for example, 750.degree. C. Therefore, short-time annealing which is basically conducted by the continuous annealing method, when applied to such semi-processed steel strip, could not stably provide superior magnetic properties.